Storage cell with variable power level

ABSTRACT

THIS SPECIFICATION DESCRIBES A SEMICONDUCTOR STORAGE CELL FOR USE IN MONOLITHIC MEMORIES. THE STORAGE CELL HAS TWO CROSSCOUPLED FET&#39;&#39;S WHICH FUNCTION AS THE STORAGE ELEMENTS OF THE CELL. THE CROSSCOUPLED FET&#39;&#39;S ARE ADDRESSED POWERED THROUGH INPUT-OUTPUT FET&#39;&#39;S WHEN THE CELL IS INTERROGATED FOR READING. WHEN THE CELL IS NOT BEING SO INTERROGATED, THE CROSSCOUPLED FET&#39;&#39;S ARE SUPPLIED POWER FROM A SOURCE WHICH IS CONNECTED TO EACH OF THE CROSSCOUPLED FET&#39;&#39;S BY A SEPARATE LOAD FET. THE GATES OF THOSE LOAD FET&#39;&#39;S ARE BIASED SO THE LOAD FET&#39;&#39;S SUPPLY CHARGE TO THE CROSSCOUPLED FET&#39;&#39;S WHILE THE STORAGE CELL IS NOT BEING INTERROGATED BUT DRAW CHARGE FROM THE CROSSCOUPLED FET&#39;&#39;S WHEN THE CROSSCOUPLED FET&#39;&#39;S ARE ADDRESSED FOR READING. BY BIASING THE LOAD FET&#39;&#39;S IN THIS   MANNER, THE POTENTIAL ON THE DRAIN CAN BE REDUCED SO AS TO REDUCE THE OVERALL POWER DISSIPATION OF THE STORAGE CELL.

1: limited States atent l 13,588,846

[72] Inventors Richard H. Linton; OTHER REFERENCES George Y. Sonoda. Poughkeepsie. 'N-Y' IBM Technical Dis. Bul Vol. 10 011 Apr. 1968 pp. 1715- 1 1 PP No 731528 1716 Associative Memory Cell" Behnke 1 Filed 1968 Integrated Fast-Read, Slow Write Memory Cell Keller, 1 1 Patented June 1971 IBM Tech. Dis. Bul Vol. 1001 June, 1967 [73] Assignee International Business Machines Corporation Primary Exammcr- Terrell Fears Armonk NY Attorneys-Hamfin and Jancin and James E. Murray [54] STORAGE CELL WITH VARIABLE POWER LEVEL ABSTRACT: This specification describes a semiconductor 4 Claims, 3 Drawing Figs storage cell for use in monolithic memories. The storage cell has two crosscoupled FETs which function as the storage ele- 1 1 Cl 340/173i ments of the cell. The crosscoupled FETs are address 307 307/279 powered through input/output FETs when the cell is inter- [Sl] Int.Cl Gl1cl1/40, rogated for reading w the Ce" is not being so imcp H031: 3/286 rogated, the crosscoupled FETs are supplied power from a 1 Field Search I a 340/173; source which is connected to each of the crosscoupled FETs 3070381279 by a separate load FET. The gates of those load FETs are 7 biased so the load FETs su 1 char e to the crosscou led [56] Reierences cued FETs while the storage cell i fi t being interrogated but d i'aw UNITED STATES PATENTS charge from the crosscoupled FETs when the crosscoupled 3,067,336 12/1962 Eachus 340/173 FETs are addressed for reading. By biasing the load FETs in 3,284,782 11/1966 Burns 340/173 this manner, the potential on the drain can be reduced so as to 3,309,534 3/1967 Yu 340/173 reduce the overall power dissipation of the storage cell.

ov- A WRITE"0" ONLY Q I OI BIT LINE BIT LINE :1: 2:11: c1 02 16 r 1 WORD LlNE READ 0R WRITE PATENTEUJUN28|97I 8588.848

SHEET 1 OF 2 FIG. 1

WORD LINE +v5 I L READ 0R WRITE POTENTIAL OF BOTH +v2 BIT LINES WORD LINE v 3 POTENTIAL 0 f 1 "0" BIT LINE m SENSE CURRENT 1 E "1" an LINE IL SENSE CURRENT I mEE. BIT SENSE n SENSE CURRENT- INVENTORY) RICHARD H. LIN I OH GEORGE Y SUNODA ATTORNEY PATENIFLII .JUN28 III:

SHLU 2 [II 2 +vI N n N M v3 II CELL CELL -I CELL I 'L woRD DRIVER z; HI 1 I CELL CELL CELL w0RD DRIVER L Hum.

BI 80 B1 B0 1 BO BIT DRIVER BIT DRIVER BIT DRIVER SENSE AMP SENSE AMP SENSE AMP B1 I B0 I WR|TE"I" WR|TE"0" STORAGE CELL WITH VARIAWLE POWER LEVEL BACKGROUND OF THE INVENTION I the heating problems become more critical and very sophisticated and expensive cooling apparatus must be used in order to maintain the memory at an operating temperature level. At even higher bit densities, it becomes impossible to cool the chips with conventional cooling systems. For these reasons dissipation of heat by the cells materially adds to the cost of monolithic computer memories and also is a limiting factor on the speed of operation of the memory and the size of the memory. Therefore, it is desirable to reduce the dissipation of heat by the cells. One method of doing this is to power storage cells at two levels. That is, supply one level of power to the storage cells while the storage cells are being addressed for reading and/or writing and supply another lower level of power to the storage cells while the storage cells are merely storing information.

SUMMARY The present invention relates to such bilevel powered storage cells and provides a means for reducing the potential supplied to the cells while the cells are merely storing information thereby materially decreasing the power dissipated by the storage cells. As in most monolithic memories the storage cells of the present invention each have two semiconductor devices which are crosscoupled to form a bistable circuit; These storage F ET 5 are address powered through input/output FETs when the cell is interrogated for reading. When the cell is not so interrogated, the storage elements are supplied power from a source which is connected to each of the crosscoupled storage elements by separate load FETs. The gates of these load FETs are biased at a potential which is higher than that of the drain so that the load FETs supply charge to the crosscoupled FETs while the storage cell is not being interrogated but draw charge from the crosscoupled FETs when the crosscoupled FETs are addressed. By biasing the load FETs in this manner, the potential at the drain is reduced to reduce the power dissipation of the storage cell.

Therefore, it is an object of the present invention to provide storage cells which can be fabricated into monolithic memories.

It is also an object of this invention to provide a storage cell which dissipates very little power.

It is a further object of this invention to provide storage cells which operate at two different power levels.

DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiment of the invention as illustrated in the accompanying drawings of which:

FIG. l is a schematic of a storage cell of the present invention;

FIG. 2 are curves produced by reading the information stored in the storage cell; and

FIG. 3 is a schematic illustrating how the storage cell of the present invention may be hooked into matrices so as to form memory arrays.

Referring to FIG. I, the sources of the crosscoupled FET devices Oil and 02 are connected to the grounded terminal of a 2-volt power supply while the drains of both the FET devices Q1 and Q2 are connected through separate load devices Q3 and 0% to the positive terminal of this same power supply. Thus devices Q1, Q2, Q3 and Q4 constitute a bistable Schmidt trigger circuit in which devices Q1 and Q2 are the active crosscoupled devices of the trigger and the devices Q3 and Q4 are the loads for the active devices.

Information is stored in this bistable trigger circuit in the form of binary 1"s and 0"s. A binary l is stored in the circuit when device Oil is conducting and device O2 is off and a binary 0" is stored in the circuit when device O2 is conducting and device O1 is off. v

For the purpose of reading or changing the information stored in the bistable trigger circuit, FET device Q5 couples node A of the trigger to the l T bit sense terminal 12 and PET device Q6 couples node B of the trigger to the 0" bit sense terminal 14. The gates of the PET devices Q5 and Q6 are connected together and to the word line tenninal 16 for the cell so that the potentials at node A and B can both be read upon application of a single read pulse to the word line terminal 16. As will be seen later, the signals produced at the l and 0 bit sense terminals 12 and 14 as a result of the application of this read pulse are fed into a differential amplifier and compared to see ifa 1" or 0" is stored in the cell.

Previously, we mentioned that the potential Vl or the potential at the drains of PET devices Q3 and Q4 was two volts. In accordance with the present invention, the potential V1 on the drains of devices Q3 and O4 is lower than V2 by at least the operating threshold potential of devices Q3 and Q4. This enables the storage cell to operate on the low 2-volt potential difference between V1 and ground. If the potential V1 on the drains of devices Q3 and Q4 were the same as the potential V2 much larger potential difference, between V1 and ground, would have to be used in order to maintain the information in the storage cell while the cell is not being addressed.

Let us now assume that the cell is not being addressed either for writing or reading and a l is stored in the cell. With a 1" stored in the cell, device O1 is conducting and device O2 is off. Conduction through device Q1 lowers node A to approximately ground potential while node B remains approximately at 2 volts or at the potential V1. Since the cell is not being addressed, devices Q5 and Q6 are biased off by application of the potential V2 to the bit line terminals 12 and 14 and ground potential to the word line terminal 16. Thus the current to maintain device 01 conducting flows through devices Q3 and Q4.

The magnitude of the potentials V1 and V2 and the impedance of devices 03 and Q4 are selected so that this current is the minimum necessary to maintain the state of the trigger or in other words the minimum necessary to maintain device OH on and device Q2 off as a result of the corsscoupling of the drains of devices Q1 and Q2. However, the potential at nodes A and B is not sufficient to permit nondestructive reading of the information stored in the storage cell. To prevent the destruction of the information stored in the cell during reading, the potential of the nodes A and B is raised by excitation supplied to the nodes A and B from the bit terminals 12 and M. For this purpose the potential at the bit terminals 12 and M is maintained at +V2 (approximately 4 volts). Then devices Q5 and Q6 are turned on by a positive interrogation pulse V3 applied to the word tenninal 16. This reduces the impedance of devices Q5 and Q6 allowing current to flow to the nodes A and B from the terminals 12 and 14. As current flows from terminal 12 to the on" node A, the potential at node A rises. Similarly, as current flows from terminal 14 to the off node B, the potential at node B rises. These currents flow along bit lines 18 and 20 to a differential sense amplifier 22 where they are subtracted to provide a differential sense current which identifies the information stored in the cell. FIG. 2 shows the sequence of voltages and currents occurring during a read l cycle.

As the "off" node B rises in potential during a read cycle, current starts flowing through device Q4 away from node B towards the source V1. This current through device Q4 subtracts from the differential sense current. However devices Q3 and Q4 are high impedance devices so that the reduction in differential sense current is less than 10 percent. Furthermore,

there is a definite advantage in using a low drain voltage on devices Q3 and Q4 although it permits current to flow away from the nodes A and B during the read and write cycles of the storage cell. The advantage is that it permits a power reduction when compared with the cell in which a common gate and drain power supply for the load devices is provided. The voltage V1 is chosen so it is at least one threshold level below V2 and as such the power dissipation by the cell'is reduced by 2.3 times over the same cell with the common gate and drain power supply. This power reduction figure is based on the use of N channel enchancement mode MOSFET devices as the devices Q1, Q2, Q3, Q4, Q5 and Q6.

So far we have described how to read the information stored in the storage cell. To change the information stored in the cell or in other words to write a into the cell, the potential of0" bit terminal 14 is lowered to ground potential while the potential on the l bit terminal is maintained at +V2. Then a positive interrogation pulse is applied to the word terminal 16 to turn devices Q and Q6 on. With terminal 14 biased at ground potential capacitor C2 discharges rapidly towards ground potential through device Q5. As node B drops from +Vl towards ground potential. the potential at node A starts rising starting a regenerating action that ends in device Q2 being turned on the device Q1 being turned off. Device Q5 and Q6 may then be turned off leaving the cell in its 0" storage state. To switch from the 0" storage state to the l storage state a similar process is employed except this time the potential at terminal 12 is decreased to lower the voltage at node A while devices Q5 and Q6 are conducting. This will turn device Q2 off which raises the voltage at node B and allows device Q1 to go on. Multiplicity of the above described cells can be coupled together as shown in FIG. 3 and used to form matrices that perform memory functions.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

We claim:

1. In a storage cell having a pair of crosscoupled FET devices each connected to a separate load FET device for the receipt of driving potential and to bit and sense lines by an input/output FET device to permit the cell to be interrogated and sensed nondestructively, the improvement which comprises:

a. first voltage source means coupled through the controlled terminals of said load FET devices to the crosscoupled FET devices for supplying a first voltage to said bistable circuit, said voltage being sufficient to maintain the state of the bistable circuit while the cell is not being interrogated but being insufficient to permit the cell to be interrogated for reading;

b. second voltage source means coupled to the gates of the load FET devices for supplying a second voltage to the gates of said load FET devices, said second voltage being at least one threshold level greater in magnitude than the first voltage to permit the first voltage to retain the state of the cell at a potential which is smaller in magnitude than would be required to retain the state of the cell if the first and second voltages were equal; and

c. third voltage source means coupled to the bit lines for supplying a third voltage to the bistable circuit through said input/output FET devices only while the bistable circuit is being interrogated, said third voltage being larger in magnitude than said first voltage and being sufficient to permit the data in the storage cell to be sensed nondestructively and also sufficient to cause current to pass through the load FET devices in a direction opposite to that which the current flows when the storage cell is not being interrogated whereby the power dissipated by the storage cell is reduced.

2. The storage cell of claim 1 wherein the first of the controlled terminals of each of the load FET devices is connected to the other terminal of the first source of potential, the other controlled terminal of each of the load FET devices is connected to a different one of the gates of the crosscoupled F ET devices, and the gates of the load FET devices are biased at least one threshold level greater in magnitude than the potential at said first of the control terminals of the load FET DEVICES.

3. The storage cell of claim 2 wherein the first of the controlled terminals of each of the input/output FET devices is connected to a different one of the gates of the crosscoupled FET devices, the second of the controlled terminals of each of the input/output FET devices is connected to a different one of the sense lines and the gates of the input/output FETs are connected to the interrogation line.

4. The storage cell of claim 3 wherein the impedance of the load FET devices is high enough to limit the decrease in differential sense current in the bit sense lines to no more than 10 percent of the highest value. 

